Environmental constraints prohibit discharge of untreated acidic cooling pond water produced in phosphoric acid plants. During periods of heavy rainfall, run-off mixes with cooling pond water, which then must be processed by conventional water treatment plants before being discharged, considerably increasing the cost of plant operation.
Consumption of excess water in the phosphate rock grinding circuit of the phosphoric acid plant is one solution to the problem of excess pond water disposal. However, since excess water is mixed with acidic pond water, internal use of the water in the ball mill is impractical because the acidic pond water is highly corrosive.
At least two processes, the LANG process and the C.F. Industry process, are presently known for neutralizing pond water for use in the phosphate rock grinding circuit. In both, pond water is partially neutralized with ammonia. These two processes, however, are not completely satisfactory because the resulting rock-water slurry used in the ball mill is still somewhat corrosive, probably because of the "salt effect" of the ammonium ion remaining in the solution. Moreover, the ammonium ion in the remaining solution is a problem in subsequent processing because it causes excessive precipitation of sludge at the concentrator stage of the phosphoric acid unit.
An economical process, which avoids the corrosion problems of prior art processes, is therefore needed for treating excess cooling pond water to make it suitable for initial use in the grinding circuit of a phosphoric acid plant.